The present invention relates generally to formation of a fine pattern with direct electron beam delineation, and more particularly to an exposure evaluating method of obtaining an electron beam scattering intensity distribution for a proximity effect correction in the fine-pattern formation.
In performing a so-called electron beam exposure, an electron beam scattering occurs in a resist, by which scattering the exposure area becomes wider than the electron beam illumination area so as to enhance affection of the scattering to make it difficult to obtain a pattern with a desirable dimension. This is referred to as proximity effects. Thus, formation of a fine pattern requires the proximity effect correction.
Generally, the electron beam scattering intensity distribution can be expressed by the following double Gaussian distribution under the condition that in a coordinate system the center point is taken as xl and the scattering intensity is E(r). ##EQU1## where .alpha. represents the spread of the forward scattering occuring in the resist, .beta. designates the spread of the back scattering generated by reflection from the substrate, and depicts the reflection coefficient of the back scattering.
When a pattern represented by a region S1 is exposed with an exposure dose (quantity of illumination) Q.sub.E, the absorbed dose Q(r) in the coordinate R can be expressed in accordance with the following equation. EQU Q(r)=.sqroot..sub.s1.spsb.Q.sub.E .multidot.E(r-x1)d.sup.2 x1 . . . . . . . . . . . . (2)
Secondly, when this pattern is exposed with an exposure dose Q.sub.E1 and the resist in the coordinate r is first removed after development, the dissolved absorbed dose Qc of the resist can be diven in accordance with the following equation. EQU Qc=.sqroot..sub.s1.spsb.Q.sub.E1 .multidot.E(r-x1)d.sup.2 x1 . . . . . . . . . . . . (3)
When carrying out the proximity effect correction, for forming an expected pattern, after obtaining the exposure region S1 and the exposure dose Q.sub.E1 to satisfy the condition that the absorbed dose Q(r) in the equation (2) is greater than the dissolved absorbed dose Qc in the region including the expected pattern but smaller than the dissolved absorbed dose Qc, the exposure region S1 is exposed with the exposure dose Q.sub.E1 so as to result in execution of the proximity effect correction. Although the above description has been made in terms of using a positive resist, with respect to a negative resist, the proximity effect correction can be effected by a similar manner in which the resist-removing portions is considered as resist-remaining portions.
Generally, the proximity effect correction is made by setting a number of evaluation points on the border line of the expected pattern and setting up an equation so as to satisfy the equation (3) on all of the evaluation points, thereby obtaining the exposure region and the exposure dose which satisfy the above-mentioned condition as the solution of the set-up equation. At this time, in the equation (3), the forward scattering spread .alpha., the back scattering spread .beta., the reflection coefficient .eta. and the resist dissolved absorbed dose Qc becomes parameters depending upon a process. Here, these parameters depend upon the material of the resist, development condition of the resist, material of the substrate, acceleration voltage of the electron beam and others and are hence required to be renewed if at least one of them changes. Further, of the aforementioned parameters, the resist dissolved absorbed dose Qc can be easily obtained because it can be defined as the exposure dose at the time that the film thickness of the resist first becomes zero when exposing a pattern extremely wider than the scattering length of the electron beam. Accordingly, it is required to easily obtain .alpha., .beta., and .eta. of the aforementioned parameters. In addition, in the equation (3), .alpha. is generally known to be about 0.1 to 0.3 .mu.m and the correction effect can be obtained even if approximating as .alpha.=0, and therefore there is no problem in terms of obtaining only .beta. and .eta..
A description will be made hereinbelow with respect to known methods of obtaining the parameters .alpha., .beta., and .eta..